This disclosure relates to using neutron-induced gamma-ray spectroscopy to distinguish between nuclei of an element (e.g., carbon) located nearer to a detector of a downhole tool from nuclei of the element (e.g., carbon) located farther from the detector of the downhole tool in the well.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as an admission of any kind.
Producing hydrocarbons from a wellbore drilled into a geological formation is a remarkably complex endeavor. In many cases, decisions involved in hydrocarbon exploration and production may be informed by measurements from downhole well-logging tools that are conveyed deep into the wellbore. The measurements may be used to infer properties and characteristics of the geological formation surrounding the wellbore. The discovery and observation of resources using downhole techniques generally takes place down in the wellbore with sensors. These sensors may be a part of a tool-string that may be attached to a drill or other downhole device.
One particular type of sensor uses a method of direct carbon measurement using neutron-induced gamma-ray spectroscopy. This method may be used to estimate the prolificacy of oil or other carbon-based resource of interest in the area. This technique may include a neutron source and one or more gamma-ray detectors. In general, this technique is practiced by emitting high-energy neutrons into the environment and detecting gamma-ray radiation from the surrounding carbon nuclei. The high-energy neutrons emitted into the environment may collide with and/or scatter off a carbon nucleus, thus causing gamma-ray emission of a certain energy. These gamma-rays may then be detected and the information processed to reveal the carbon in the environment.
The method of neutron-induced gamma-ray spectroscopy may, however, be subject to inaccuracies due to the carbon in the wellbore itself. Carbon in the wellbore, such as oil or oil-based mud, may appear in the same signal measuring the carbon in the surrounding geological formation. Because the wellbore carbon and the carbon in the geological formation are measured at the same time, it may be difficult to accurately estimate the amount of carbon in a reservoir in the geological formation, thus causing inaccuracy in the resource's volume estimate.